Method for producing oxidation-resistant high cr ferritic heat resistant steel

ABSTRACT

A high Cr ferritic heat-resistant steel (Cr content of 15 mass % or less), which, at least a region 10 μm in depth as measured from the surface has a worked texture comprising elongated ferritic grains or a fine crystal grain texture comprising ferritic grains of 3 μm or less in diameter, and has a protective coating film on the surface; it is a high Cr ferritic heat-resistant steel improved in oxidation resistance without causing drop in high temperature strength or in toughness.

TECHNICAL FIELD

[0001] The invention of this application relates to a method forproducing ferritic heat-resistant steel for use in boilers of thermalpower plants, chemical industry apparatuses, and the like that areoperated at high temperatures and under low oxygen partial pressureatmospheres.

BACKGROUND ART

[0002] In our country, about 60% of the total electric power demand issupplied by thermal electric power plants using fossil fuels; however,large amount of carbon dioxide is being emitted by the combustion offossil fuels.

[0003] On the other hand, higher power generation efficiency is stronglydemanded on thermal power plants in view of the emission regulations oncarbon dioxide for the prevention of global warming or from theviewpoint of effective use of resource energy. Heat resistant steel andheat resistant alloys have been used as materials resisting to hightemperatures and high pressures at thermal power plants; in the case ofusing such heat resistant steel and heat resistant alloys under theatmosphere, dense oxide coating film forms on the surface to function asprotective layers.

[0004] However, in the presence of high temperature water vapor underlow oxygen partial pressure atmospheric condition as such in thermalpower plant boilers, the apparatuses used under such conditions suffergreater oxidation damage as compared with those used under atmosphericconditions, because the supply of oxygen is not sufficient to formprotective coating films.

[0005] In general, Cr-containing heat resistant steel and heat-resistantalloys having a Cr content of 25 mass % or more show excellentoxidation-resistance because oxidation-resistant protective film isformed even under high temperature water vapor atmospheric condition.

[0006] Furthermore, in the case of chromium (Cr)-containing heatresistant steel and heat-resistant alloys having a Cr content of orabout 20 mass %, it is possible to modify the surface of the basematerial by applying a mechanical treatment such as shot peening, or toform an oxidation-resistant protective coating film by a method such ascrystal grain refining treatment and the like.

[0007] However, in the case of a high Cr ferritic heat-resistant steelcontaining 15 mass % or less of Cr, the Cr content is too low to supplythe Cr oxide necessary for forming the oxidation-resistant protectivecoating film. Accordingly, as methods for ameliorating the oxidationresistance of a high Cr ferritic heat-resistant steel containing mass %or less of Cr, attempts such as increasing chromium (Cr) or silicon(Si), or adding palladium (Pd) or platinum (Pt), etc., have been made(see, for example, literatures 1˜4); however, they could not avoid thedeterioration of the quality of material or the increase in cost due tothe addition of elements such as palladium (Pd), platinum (Pt), etc. Asdescribed above, an effective method for improving the oxidationresistance of a high Cr ferritic heat-resistant steel containing 15 mass% or less, for instance, 9 to 12 mass %, is yet to be realized.

[0008] Literature 1; JP-A-2002-69531

[0009] Literature 2; JP-A-2001-192730

[0010] Literature 3; JP-A-11-61342

[0011] Literature 4; JP-A-10-287960

[0012] The invention of the present application has been made under suchcircumstances, and the objectives thereof are to overcome the problemsof the conventional techniques and to provide a high Cr ferriticheat-resistant steel on which an oxidation-resistant protective coatingfilm is formed even in the presence of high temperature water vaporunder low oxygen partial pressure atmospheric condition.

DISCLOSURE OF THE INVENTION

[0013] As a solution for the aforementioned problems, the invention ofthe application provides the invention as follows.

[0014] That is, the invention of the present application provides,firstly, a steel characterized in that it is a ferritic heat-resistantsteel containing 15 mass % or less of Cr, in which at least the regionup to 10 μm defined by surface depth is made of a worked texturecomposed of extended ferritic grains or superfine texture composed offerrites 3 μm or less in grain diameter, and having a protective coatingfilm on the surface. Secondly, it provides a steel characterized in thatthe shorter diameter of the extended ferritic grains is 5 μm or less;and thirdly, it provides a steel characterized in that the shorterdiameter of the extended ferritic grains is 3 μm or less, or the graindiameter of the ferrites is 1 μm or less.

[0015] The invention of the present application provides, fourthly, aproduction method for a steel of one of claims 1 to 3, in which workingis applied in the temperature range of from 400 to 800° C. to form aworked texture or superfine ferrite grain texture at least in the regionup to 10 μm defined by surface depth, and pre-oxidation treatment isapplied to form a protective coating film; fifthly, it provides aproduction method in which the working degree on applying working is 0.7or higher in true strain; and sixthly, a production method in which thepre-oxidation treatment is carried out by holding the steel for 30 to 90minutes in the temperature range of from 400 to 800° C. under theatmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a cross sectional scanning electron microscope (SEM)photograph of an intensively worked part which have been subjected tocompression working at a temperature of 500° C., followed bypre-oxidation treatment in the atmosphere at 640° C./1h, an then bywater-vapor oxidation at 650° C./100h;

[0017]FIG. 2 is a cross sectional SEM photograph of a weakly worked partwhich have been subjected to compression working at a temperature of500° C., followed by pre-oxidation treatment in the atmosphere at 640°C./1h, an then by water-vapor oxidation at 650° C./100h; and

[0018]FIG. 3 is side cross sectional SEM photograph of an intensivelyworked part which have been subjected to compression working at atemperature of 500° C., followed by water-vapor oxidation at 650° C./3hwithout applying pre-oxidation treatment.

BEST MODE FOR CARRYING OUT THE INVENTION

[0019] The invention of the application has the above characteristics,and the mode of carrying out the invention is described below.

[0020] First of all, in ameliorating the oxidation resistance of a highCr ferritic heat-resistant steel containing 15 mass % or less of Cr evenin the presence of high temperature water vapor or under low oxygenpartial pressure atmospheric condition, the invention of the applicationis most characterized by that the oxidation resistance is improved byapplying a specific working and heat-treatment, and not by increasingthe composition of chromium (Cr) or silicon (Si), nor by adding elementssuch as palladium (Pd) or platinum (Pt) and the like. Accordingly, thehigh Cr ferritic heat-resistant steel obtained by the method forimproving the oxidation resistance according to the invention of theapplication possesses an advantage in that the physical properties andthe chemical properties of the initial composition remain without beingimpaired.

[0021] In general, in case the Cr content exceeds 15 mass %, asdescribed above, the surface layer of the steel material can be modifiedby applying mechanical treatment such as shot peening and the like, oran oxidation-resistant protective coating film can be formed by applyingrelatively mild crystal grain refining treatment with large grains about10 to 50 μm in particle diameter, however, in high Cr ferriticheat-resistant steel containing 15 mass % or less Cr, suchoxidation-resistant protective coating film cannot be formed even ifsuch treatment should be employed. The reason for the above is because,in case the Cr content is 15 mass % or less, Cr necessary for formingthe protective coating film containing Cr₂O₃ as the principal componentcannot be sufficiently and uniformly diffused even if the crystal grainrefining treatment for obtaining crystals about 10 to 15 μm in graindiameters should be applied. Accordingly, oxidation-resistant protectivecoating film cannot be formed in the presence of water vapor under hightemperatures.

[0022] Thus, the keys of the invention of the present application are toaccumulate strain energy at high levels by applying warm intensiveworking to the high Cr ferritic heat-resistant steel, or to form finetexture consisting of crystal grains 2 μm or less in grain diameter. Inthe invention of the application, the reason for accumulating strainenergy at high levels or for forming fine texture consisting of crystalsgrains 3 μm or less in grain diameter is because, the steel materialhaving the strain energy accumulated in high levels easily undergoesrecrystallization to form superfine grain texture. Thus, with theformation of the superfine grain texture above, the grain boundary areaincreases to contribute for the diffusion acceleration of chromium (Cr).Then, by the uniform diffusion of chromium (Cr), chromium oxide (Cr₂O₃)is formed to function as an oxidation-resistant protective coating film.As described above, the strain energy is highly accumulated in theinvention of the application. Further, the formation of superfine graintexture consisting of ferrite grains 3 μm or less in grain diameter maybe considered as a mode of accumulating the strain energy at a highlevel.

[0023] By the thermomechanical treatment such as the rolling or theforging generally employed in the art, it is not possible to highlyaccumulate the strain energy or to form a fine texture consisting offerritic crystal grains 3 μm or less in grain diameter according to theinvention of the present application. In order to highly accumulate thestrain energy or to form the fine crystal grain texture consisting ofcrystal grains 3 μm or less in grain diameter, it is preferred toperform warm working treatment at a strain rate of 0.1sec⁻¹ or higherand at a working ratio (cross section area reduction ratio) of 70% orhigher. In case the working ratio is lower than 70%, the accumulation ofthe desired strain energy remains insufficient, and the generation ofprotective coating film and the use thereof cannot be fully expectedever after the pre-oxidation treatment.

[0024] Considering the warm intense working, it is preferably carriedout in the temperature range of 400 to 800° C. Further, by forming thestrain under the conditions above, it is possible to generate theelongated ferritic grains or fine grains.

[0025] As the morphology of the elongated ferritic grains, it ispreferred that it has a shorter diameter of 5 μm or less, and mostpreferably, it has a shorter diameter of 3 μm or less; otherwise, theferritic grains has a grain diameter of 3 μm or less, and mostpreferably, they are fine grains 1 μm or less in grain diameter.

[0026] As described above, the invention of the present applicationcomprises accumulating strain energy at high levels by applying warmintensive working to the high Cr ferritic heat-resistant steel, orforming fine texture consisting of crystal grains 3 μm or less in graindiameter; however, the protective coating film does not always form athigh temperatures in the presence of water vapor by simply accumulatingthe strain energy by warm intensive working or by forming fine crystaltexture. It is necessary to form sequentially thereafter the protectivecoating film by applying pre-oxidation treatment. The pre-oxidationtreatment is preferably carried out under the atmosphere or under aninert gas (rare gas or gaseous nitrogen) atmosphere containing gaseousoxygen, but more practical is to perform the treatment in theatmosphere. Further, the pre-oxidation treatment is preferably performedby heat treatment in the atmosphere at 400˜800° C. for about 30 to 90minutes.

[0027] By combining the heat treatments above, chromium (Cr) is oxidizedfor the first time, and forms Cr₂O₃ which functions as anoxidation-resistant protective coating film.

[0028] Furthermore, concerning the relation between the heatingtemperature of the pre-oxidation treatment and the average crystal grainsize, it is confirmed that, when heated under the atmosphere, the graindiameter is 0.8 μm or less for the sample held at 660° C or lower, andis 1˜2 μm for the sample held at 680˜700° C.

[0029] As described above, the invention of the application enablesforming an oxidation-resistant protective coating film on the high Crferritic heat-resistant steel having a Cr content of 15 mass % or less,on which an oxidation-resistant protective coating film had never beenformed, and this widely increases the usage of high Cr ferriticheat-resistant steel. Furthermore, the invention of the application isadvantageous in that it utilizes heat treatment, because it does notmake any changes on the composition of the high Cr ferric heat-resistantsteel. Moreover, since the protective coating film is thin and istightly adhered, it hardly peels off; it thereby exhibits effect ongreatly reducing the risk of causing clogging of the piping due to thepeeled off scales, or of wearing the turbine blades.

[0030] It should be noted that the high Cr ferritic heat-resistant steelof the invention of the application includes those of variouscompositions containing 15 mass % or less of Cr. For instance, there areincluded those containing 7 mass % to 15 mass % of Cr.

[0031] Steel, such as the high Cr ferrite heat-resistant steel specifiedin ASME SA35 P91 or ASME SA 213 T91, are included. These are specifiedin the invention of the application by the general term “high ferritic(system)” steel.

EXAMPLES Example

[0032] Mod. 9Cr-1Mo steel was subjected to 70% compression working at500° C., and was cut and polished in such a manner that the fine textureregion and the worked texture region should be exposed on the surface.Then, after applying pre-oxidation treatment at 650° C. for 1 h in theatmosphere, the test specimen was oxidized in water vapor at 650° C./100h. FIG. 1 shows the cross section SEM photograph of the intensivelyworked part. The formation of the Cr-rich (Cr₂O₃) protective coatingfilm (0.1 μm or less in thickness) on the surface was observed.Furthermore, the crystal grain diameter of the ferritic grains in thefine texture region under the protective coating film was found to be1.0 μm or smaller. Further, the shorter diameter of the elongatedferrite in the worked texture region under the protective coating filmwas found to be 3 μm.

Comparative Example 1

[0033] Mod. 9Cr-1Mo steel was subjected to pre-oxidation treatment at680° C. for 1 h in the atmosphere, and was then subjected to water-vaporoxidation at 650° C./100 h. FIG. 2 shows the cross section SEMphotograph of the resulting product, but the growth of an Fe-rich doublelayer scale (about 60 μm in thickness) due to accelerated oxidation wasobserved. The average grain diameter of the ferritic grains was 7 μm.

[0034] By comparing the above result with that of the Example, it wasconfirmed that the high accumulation of strain energy or the formationof fine crystal texture is necessary to form a protective coating filmhaving resistance against water-vapor oxidation.

Comparative Example 2

[0035] Mod. 9Cr-1Mo steel was subjected to 70% compression working at500° C. in a manner similar to that of Example 1, and was subjected towater-vapor oxidation at 650° C./3 h thereafter without applyingpre-oxidation treatment for observation. FIG. 3 shows the cross sectionSEM photograph of the intensively worked part. The formation of doublelayer scale (about 10 μm in thickness) was observed.

INDUSTRIAL APPLICABILITY

[0036] As described in detail above, the invention of the applicationenables forming a thin and highly adhesive oxidation-resistantprotective coating film, which had been believed impossible, on the highCr ferritic steel having a Cr content of 15% or less.

1. A steel which is a ferritic heat-resistant steel containing 15 mass %or less of Cr, in which at least the region up to 10 μm defined bysurface depth is made of a worked texture composed of extended ferriticgrains or superfine texture composed of ferrites 3 μm or less in graindiameter, and having a protective coating film on the surface.
 2. Thesteel of claim 1, wherein the shorter diameter of the extended ferriticgrains thereof is 5 μm or less.
 3. The steel of claim 1 wherein theshorter diameter of the extended ferritic grains is 3 μm or less, or thegrain diameter of the ferrites is 1 μm or less.
 4. A production methodfor a steel of claim 1 which comprises applying working in thetemperature range of from 400 to 800° C. to form a worked texture orsuperfine ferrite grain texture at least in the region up to 10 μmdefined by surface depth, and by applying pre-oxidation treatment toform a protective coating film.
 5. The production method of claim 4,wherein the working degree on applying working is 0.7 or higher in truestrain.
 6. The production method of claim 4, wherein the pre-oxidationtreatment is carried out by holding for 30 to 90 minutes in thetemperature range of from 400 to 800° C.